An opinion since nobody else is offering one-

Consider if they are putting lives or property in danger. Flying not-by-the-book is one thing, flying unsafely is quite another. If you report them and the FAA violates them, you will be seriously damaging their future flying career and costing them a lot of money. So save that option as a last resort, you want to get them to fly safe not punish them.

A lot of the answer (safe or not) depends on where you are, is there radar/ATC, etc. If they are shooting a practice approach on their portable GPS in VFR weather, who cares? Maybe (s)he wants to have a way down in case they go inadvertent IMC (not a bad idea if there's no radar or ATC service where you are). But if they are doing this in real IMC on a regular basis when they should be diverting (especially with passengers) that should probably stop.

If you feel they are causing real actual danger and not just playing a bit loose with the regs, you should do something about it. First talk with those other pilots, ask if they feel his flying creates danger to people or property. If so, you should first talk to the pilot in question, and tell him you feel he needs to change his ways and WHY- don't accuse, educate. Bonus points if you can get those other pilots to do the same, that way you can peer pressure him into flying safely with no negative consequences and everybody wins. If that doesn't work or he ignores you, talk to the flying club. Don't tell them what to do about it, just say that you feel like he is flying unsafely (and why), you worry that he may be passing those bad habits on to students, and that you talked to him but he said (whatever he said). They will almost certainly do SOMEthing about it.

FAA should be your last resort. If he is really flying in a dangerous manner that causes unnecessary risk to others (not just himself) and nobody will do anything about it, then call your local FSDO. Tell them what this pilot does and why you feel it causes danger. At that point it's out of your hands.